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North Korea frequently tests missiles at times of heightened tensions with South Korea

North Korea has test-fired four short-range missiles into the sea off its west coast, say South Korean military officials.

In a statement, the Joint Chiefs of Staff (JCS) said the missiles had a range of about 140km (87 miles).

They were fired from Dongchang-ri in the north of the country on Friday afternoon local time, it said.

The North often conducts missile tests in protest at US-South Korean military drills, one of which is ongoing.

The US and South Korea say the annual exercises are for defence training purposes, but Pyongyang calls them a rehearsal for invasion. They are always a trigger for a surge in tensions between the two Koreas.

When a drill began in March, the North fired two short-range ballistic missiles, and on 13 March it fired seven ground-to-air missiles into the sea to coincide with the end of one part of the drill, Operation Key Resolve.

The current drill, Foal Eagle, is continuing.

A JCS spokesman said Friday's test "appeared to have been supervised by Kim Jong-Un", the AFP news agency reports.

The two Koreas are technically still at war as the 1950-53 conflict ended with a ceasefire, not a peace treaty.